System and method for operating an electrical energy storage system

ABSTRACT

Systems and methods for operating an electric energy storage system are described. The systems and methods include ways of coupling electric energy storage cell stacks to an electric conductor or bus. The coupling is performed to reduce current flow through contactors and to increase a life span of the contactors.

FIELD

The present description relates to a system and method for operating anelectric energy storage system. The methods and systems may beparticularly useful for electrical power systems that include aplurality of electric energy storage cell stacks.

BACKGROUND AND SUMMARY

An electrical energy storage system may include electrical energystorage cells (e.g., battery cells) that are arranged in series and inparallel. Electrical energy storage cells that are arranged in seriesincrease the voltage potential of the electric energy storage system.Electric energy storage cells that are arranged in parallel increase thepower output capacity of the electric energy storage system. Theelectric energy storage cells of one electric energy storage cell stackmay be charged to a level (e.g., voltage) such that the voltage of theone electric energy storage cell stack that includes the electric energystorage cells is lower or higher than levels of other electric energystorage cell stacks within the electrical energy storage system. Thedifference in charge stored between the electric energy storage cellstacks may cause larger amounts of current to flow from more highlycharged electric energy storage cell stacks to electric energy storagecell stacks having lower charge levels. This may increase thepossibility of degradation within the electrical energy storage system.Therefore, it may be desirable to provide a way of coupling electricenergy storage cells from different electric energy storage cell stacksin a way that may reduce the possibility of degradation within theelectric energy storage system.

The inventors herein have recognized the above-mentioned issues and havedeveloped a method for operating an electric energy storage system,comprising: sequentially closing a plurality of contactors thatselectively couple a plurality of electric energy storage cell stacks toan electric power conductor within the electric energy storage systemaccording to individual voltages of each of the plurality of electricenergy storage cell stacks via a controller.

By sequentially closing a plurality of contactors that may selectivelycouple a plurality of electrical energy storage cell stacks to anelectric power conductor within an electric energy storage systemaccording to the individual voltages of each of the plurality ofelectric energy storage cell stacks, it may be possible to provide thetechnical result of reducing current flow between the plurality ofelectric energy storage cell stacks. In one example, a contactor may beassociated with or be part of an electric energy storage cell stack in asystem that includes a plurality of electric energy cell stacks. Thecontactors associated with or included in the electric energy cellstacks may be closed from open states in a one after the other order.The order may begin by closing a first contactor that is associated withor that is part of electric energy storage cell stack that has a lowestvoltage of the plurality of electric energy storage cell stacks. Shortlythereafter, a second contactor that is associated with or part of anelectric energy storage cell that has the second lowest voltage of theplurality of electric energy storage cells may be closed so that twoelectric energy storage cell stacks are now coupled together. Couplingthe lowest voltage electric energy storage cell stack to the secondlowest voltage electric energy storage cell stack may raise the voltageof the lowest voltage electric energy storage cell stack and reducecurrent flow to the lowest voltage electric energy storage cell stack ascompared to the highest voltage electric energy storage cell stack beingcoupled to the lowest voltage electric energy storage cell stack.Accordingly, current flow inside of the electric energy storage systemmay be reduced, thereby increasing a life cycle of components within theelectric energy storage system.

The present description may provide several advantages. In particular,the approach may reduce the possibility of component degradation with anelectric power system. Further, the approach may be applied withouthaving to include function specific current limiting devices betweenelectric energy storage cell stacks. In addition, the approach may beapplied to a variety of different types of electric energy storage cellstacks.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram showing a single cell of an electric powerstorage and delivery system;

FIG. 2 is a schematic diagram of an electric power system that includesa plurality of electric power cells of the type shown in FIG. 1;

FIG. 3 shows several plots of electric energy storage cell voltagelevels during a process to electrically couple a plurality of electricenergy storage cells;

FIG. 4 shows a second example sequence for electrically coupling aplurality of electric energy storage cells in parallel; and

FIG. 5 shows a method for operating an electric power storage system.

DETAILED DESCRIPTION

The present description is related to operating an electric energystorage system (e.g., a direct current (DC) power source) as shown inFIG. 1. The electric energy storage system may store electrical energythat is generated via photovoltaic cells, hydroelectric power, windpower, or via chemical energy. The electric energy storage system mayoutput DC power that may be distributed as alternating current AC aftera conversion process. The electric energy storage system may include aniron flow device as shown in FIGS. 1 and 2. The electric energy storagesystem may combine a plurality of electric energy storage cell stacks inparallel as shown in FIG. 2. The electric energy storage system mayoperate as shown in FIGS. 3 and 4 according to the method of FIG. 5. Amethod for operating the electric energy storage system is shown in FIG.5.

Referring to FIG. 1, an example of an all iron redox flow battery (IFB)cell is shown. The IFB cell 175 is an electric energy storage cell. TheIFB cell may be supplied with plating electrolyte 160 (e.g., FeCL₂) thatis stored in plating electrolyte tank 100. The IFB may also includeredox electrolyte 161 that is stored in redox electrolyte tank 101. Theplating electrolyte and redox electrolyte may be a suitable saltdissolved in water, such as FeCl₂ or FeCl₃. Both the plating electrolyteand redox electrolyte may use the same salt at different molarconcentrations, a feature of the IFB not available in batteries withdifferent reactive compounds. Tank 100 may be in fluidic communicationwith negative reactor 122. Tank 101 may be in fluidic communication withpositive reactor 124. Electrolyte in tank 100 and negative reactor 122is in fluidic isolation from electrolyte in tank 101 and positivereactor 124. Separating the negative and positive reactors and theirrespective electrolytes is barrier 120. The barrier may embodied as amembrane barrier, such as an ion exchange membrane or a microporousmembrane, placed between the plating electrolyte and redox electrolyteto prevent electrolyte cross-over and provide ionic conductivity.

Sensors 102 and 104 may be used to determine the chemical properties ofthe electrolyte, including pH and may be embodied as an optical sensor.Probes 126 and 128 may additionally or alternatively be used todetermine the chemical properties (discussed below) of the electrolytes.Other examples may have a plating electrolyte probe, plating electrolytesensor, redox electrolyte probe, redox electrolyte sensor, or somecombination thereof. The probe may also be placed inside the reactingportion of the IFB in negative reactor 122 and positive reactor 124. Anacid additive may be stored in additional tanks 106 and 108. These maycontain different additives and be controlled by different routines. Inother examples, the IFB may also have either a positive side additive ora negative side additive and not both. The positive side additive may beaccelerated into the positive reactor 124 by positive additive pump 112;the negative additive may be accelerated into the negative reactor 122by negative additive pump 110. Alternately, the electrolyte additivesmay be pumped into tanks 100 and 101. Pumps 110 and 112 may be actuatedvia a control system 150 that is communicatively coupled to the pumps.The control system may be responsive to probe 126, probe 128, sensor102, sensor 104, or any combination thereof. Electrolyte may be pumpedto or from the negative reactor 122 by pump 131. Electrolyte may bepumped to or from the positive reactor 124 via pump 130. The IFBincludes a negative electrode 114 and a positive electrode 116.

Control system 150 may include inputs and outputs 154 (e.g., digitalinputs, digital outputs, analog inputs, analog outputs, pulse widthoutputs, etc.), a central processor 152, random-access memory 155, andread-only (e.g., non-transitory memory) 156. The controller 150 receivessignals from the various sensors shown in FIGS. 1 and 2. Controller 150employs the actuators shown in FIGS. 1 and 2 to adjust electric energystorage system operation based on the received signals and instructionsstored in memory of controller 150. For example, controller 150 mayadjust a speed of pump 130 according to output of sensor 126.

Control system 150 may also selectively activated and deactivate valveto allow de-ionized water 198 in tank 199 into positive and negativereactors 122 and 124. In one example, controller 150 may open valve 197and allow de-ionized water to flow into positive and negative reactors122 and 124 during an electric energy storage cell flushing process.

Referring now to FIG. 2, a schematic block diagram of an electric powersystem that includes a plurality of the IFB cells 175 a-175 x andcontroller 150. Controller 150 may read voltage levels of and currentflow through electric energy storage cell stacks 201-204 via sensors210. Controller 150 may also selectively operate contactors 220-223,main contactor 277, and charger 277. Controller 150 may receive inputfrom and provide output to human/machine interface 290, which may be adisplay panel, remote device, push-button panel, or other knowninterface.

IFB cells 175 a-175 x are the same as cell 175 shown in FIG. 1. Theletter designations are provided simply to identify individual electricenergy storage cells. IFB cells 175 a-175 f are arranged in a first cellstack 201. IFB cells 175 g-175 l are arranged in a second cell stack202. IFB cells 175 m-175 r are arranged in a third cell stack 203. IFBcells 175 s-175 x are arranged in a fourth cell stack 204. Although FIG.2 shows four cell stacks in electric energy storage system 200, electricenergy storage system 200 is not limited to four electric energy storagecell stacks. Rather, electric energy storage system 200 may include from1 to N electric energy storage cell stacks, where N is an integernumber. Further, each electric energy storage cell stack shown in FIG. 2includes six electric energy storage cells (e.g., 175 a-175 f). However,electric energy storage system 200 is not limited to six electric energystorage cells in each electric energy storage cell stacks. Rather,electric energy storage system 200 may include from 1 to M electricenergy storage cells in an electric energy storage cell stack, where Mis an integer number. Each of electric energy storage cells 175 a-175 xincludes a positive side 116 and a negative side 114.

Each electric energy storage cell stacks 201-204 includes a contactor220-223 for selectively individually coupling and decoupling electricenergy storage cell stacks 201-204 to electric power conductor or bus260. Contactor 220 includes a first side 220 a, which is directlycoupled to electric power conductor 260, and a second side 220 b, whichis directly coupled to electric energy storage cell stack 201. Likewise,contactors 221-223 include first side's 221 a-223 a, which are directlycoupled to electric power conductor 260, and second side's 221 b-223 b,which are directly coupled to electric energy storage cell stacks202-204. Contactors 220-223 may be open (e.g., not allowing current toflow through the contactor) when electric energy storage system 200 isdeactivated. Further, contactors 220-223 may be individually opened andclosed (e.g., allowing current flow through the contactor) toselectively electrically isolate selected electric energy storage cellstacks 201-204 from electric power conductor 260 when one or more ofelectric energy storage cells 175 a-175 x are flushed with de-ionizedwater. Contactors 220-223 may be selectively opened and closed viacontroller 150.

Charger 277 may supply electrical charge to electric power conductor 260when commanded by controller 150. Electrical power conductor 260 maydistribute the electrical charge to electric energy storage cell stacks201-204 when contactors 220-223 are closed. Further, charger 277 maysupply electrical charge individually to individual electric energystorage cell stacks 201-204. For example, charger 277 may only chargeelectric energy storage cell stack 201 when contactor 220 is closed andcontactors 221-223 are open. In another example of individually chargingelectric energy cell stacks, charger 277 may only charge electric energystorage cell stacks 201 and 204 when contactors 220 and 223 are closedand contactors 221 and 222 are open. Charger 277 may be selectivelyactivated to supply charge and deactivated to cease supplying charge viacontroller 150. Charger 277 may also be commanded to supply charge untilelectric power conductor 260 and electric energy storage cell stacksthat are electrically coupled to electric power conductor 260 are atcharge or voltage levels as requested by controller 150.

Electric energy storage system 200 also includes a main contactor 277that may be opened and closed via controller 150. Main contactor 277 maybe closed to electrically couple electric power conductor 260 toexternal electric energy sources (e.g., photovoltaic cells, windturbines, hydroelectric generators, etc.) 279 and electrical energyconsumers (e.g., house hold appliances, industrial motors, vehiclepropulsion sources, etc.) 278. Main contactor 277 may be opened toelectrically isolate IFB cell electric energy power conductor 260 fromelectrical energy sources 279 and electrical energy consumers 278.Electrical energy sources 279 and electrical energy consumers 278 areexternal to electric energy storage system 200.

Thus, the system of FIGS. 1 and 2 provides for an electric power system,comprising: a plurality of electric energy storage cell stacks, each ofthe plurality of electric energy storage cell stacks including acontactor, the contactor selectively coupling one of the plurality ofelectric energy storage cell stacks to an electric power conductor inthe electric power system; and a controller electrically coupled to thecontactor of each of the plurality of electric energy storage cellstacks. The electric power system further comprises executableinstructions stored in non-transitory memory of the controller tosequentially couple a plurality of electric energy storage cell stacksto the electric power conductor within the electrical energy storagesystem according to individual voltages of each of the plurality ofelectric energy storage cell stacks. The electric power system furthercomprises executable instructions stored in non-transitory memory of thecontroller to group each of the plurality of electric energy storagecell stacks into more than one group of electric energy storage cellstacks according to voltages of each of the plurality of electric energystorage cell stacks.

In some examples, the electric power system further comprises additionalinstructions to sequentially couple each of the more than one group ofelectric energy storage cell stacks to the electric power conductorwithin the electrical energy storage system according to voltages ofelectric energy storage cell stacks included in the more than one groupsof electric energy storage cell stacks. The electric power systemfurther comprises a charger coupled to the electric power conductor. Theelectric power system further comprises executable instructions storedin non-transitory memory of the controller to charge one or more of theplurality of electric energy storage cell stacks.

Referring now to FIG. 3, a prophetic example for operating an electricenergy storage system according to the method of FIG. 5 is shown. FIG. 3includes five plots that show electric energy storage system operatingconditions during a sequence where the electric energy storage system isactivated. Each of the five plots show operating states of the sameoperating conditions as shown in each of the other four plots.Therefore, for the sake of brevity, the first plot from the top of FIG.3 is described in detail, and then only the differences between thefirst plot and the remaining plots are explained.

In the example of FIG. 3, the voltages of electric energy storage cellstacks with closed contactors are within a threshold voltage of theelectric energy storage cell stack with an open contactor and a lowestvoltage of electric energy storage cell stacks that have opencontactors. Therefore, the electric energy storage cell stacks that haveclosed contactors are not charged in between the time when a contactorof a first electric energy storage cell stack is closed and when acontactor of a second electric energy storage cell stack is closed.During such conditions, the contactors of the first electric energystorage cell stack and the second electric energy storage cell stack maybe closed sooner such that there may be less time between when thecontactor of the first electric energy storage cell stack is closed andwhen the contactor of the second electric energy storage cell stack isclosed since there is no charging between contactor closings.

The first plot 300 from the top of FIG. 3 is a plot of electric energystorage cell stack voltage versus electric energy storage cell stacknumber. In this example, the electric energy storage system includes anactual total number of four electric energy storage cell stacks, whichare numbered 1-4. The electric energy storage system cell stacks arearranged in the system shown in FIGS. 1 and 2. Of course, a system withfour electric energy cell stacks is not the only configuration that themethod of FIG. 5 applies to and it should not be considered as limitingthe scope of this disclosure. The first plot shows voltages of theelectric energy storage cell stacks (e.g., as determined at an electricenergy storage cell with a highest potential voltage in the electricenergy storage cell stack) for an electric energy storage system whereall contactors of the electric energy storage cell stacks or allcontactors directly electrically coupled to the electric energy storagecell stacks are in an open state. Thus, the first plot shows opencircuit voltages for electric energy storage cells 1-4. The verticalaxis represents electric energy storage cell stack voltage and electricenergy storage cell stack voltage increases in the direction of thevertical axis arrow. The horizontal axis shows the numbers of theelectric energy storage cell stacks.

The voltage level of each electric energy storage cell stack isdifferent from the voltage level of the other electric energy storagecell stacks. In particular, electric energy storage cell stack number 2has the highest voltage 351. Electric energy storage cell stack number 3has the next highest voltage 352 and electric energy storage cell stacknumber 1 has the next highest voltage 350 after electric energy storagecell stack number 3. Electric energy storage cell stack number 4 has thelowest voltage 353. The differences in electric energy storage cellstack voltages may be attributed to the efficiencies of the individualelectric energy storage cell stacks and/or losses of the individualelectric energy storage cell stacks.

The second plot 302 from the top of FIG. 3 is a plot of electric energystorage cell stack voltage versus electric energy storage cell stacknumber when the contactor of electric energy storage cell stack number 4is closed. The contactor of electric energy storage cell stack number 4is closed because electric energy storage cell stack number 4 has alowest voltage of the four electric energy storage cell stacks. Closingthe contactor of electric energy storage cell stack number 4 does notchange the voltage levels of electric energy storage cell stack numbers1-3 because closing the contactor electrically couples electric energystorage cell stack number four to the electric power conductor or bus260 while no other electric energy storage cell stacks are coupled tothe electric power conductor or bus 260 and while main contactor 277 isopen. The voltage levels of electric energy storage cell stacks 1-4remain at their prior levels shown in the first plot from the top ofFIG. 3.

The third plot 304 from the top of FIG. 3 is a plot of electric energystorage cell stack voltage versus electric energy storage cell stacknumber when the contactors of electric energy storage cell stack numbers4 and 1 are closed. The contactor of electric energy storage cell stacknumber 1 is closed after the contactor of electric energy storage cellstack number 4 is closed because electric energy storage cell stacknumber 1 has a next lowest voltage of the four electric energy storagecell stacks at the time when all contactors of all electric energystorage cell stacks were open. Closing the contactor of electric energystorage cell stack number 1 causes the voltage of electric energystorage cell number 4 (353) to increase and the voltage of electricenergy storage cell number 1 (350) to decrease. The voltage of electricenergy storage cell number 4 (353) increases and the voltage of electricenergy storage cell number 1 (350) decreases because electrical chargeis transferred from electric energy storage cell stack number 1 toelectric energy storage cell stack number 4. The combined voltage ofelectric energy storage cell stack number 1 and electric energy storagecell stack number 4 raises the voltage of electric energy storage cellstack number 1 (350) so that when electric energy storage cell number 2is eventually electrically coupled to electric energy storage cellnumber 4, current flow into electric energy storage cell stack number 4may be reduced as compared to if electric energy storage cell stacknumber 2 where simply coupled to electric energy storage cell stacknumber 4. Consequently, a peak amount of current flow into electricenergy storage cell stack number four may be reduced. The voltage levelof electric energy storage cell stack numbers 2 and 3 remain unchangedbecause the contactors of electric energy storage cell stack numbers 2and 3 remain open.

The fourth plot 306 from the top of FIG. 3 is a plot of electric energystorage cell stack voltage versus electric energy storage cell stacknumber when the contactors of electric energy storage cell stack numbers4, 1, and 3 are closed. The contactor of electric energy storage cellstack number 3 is closed after the contactor of electric energy storagecell stack number 1 is closed because electric energy storage cell stacknumber 3 has a next lowest voltage of the four electric energy storagecell stacks at the time when all contactors of all electric energystorage cell stacks were open. Closing the contactor of electric energystorage cell stack number 3 causes the voltage of electric energystorage cell numbers 4 (353) and 1 (350) to increase and the voltage ofelectric energy storage cell number 3 (352) to decrease. The voltage ofelectric energy storage cell numbers 4 (353) and 1 (350) increases andthe voltage of electric energy storage cell number 3 (352) decreasesbecause electrical charge is transferred from electric energy storagecell stack number 3 to electric energy storage cell stack numbers 1 and4. The combined voltage of electric energy storage cell stack number 3and electric energy storage cell stack numbers 4 and 1 raises thevoltage of electric energy storage cell stack numbers 1 (350) and 4(353) so that when electric energy storage cell number 2 is eventuallyelectrically coupled to electric energy storage cell number 4, currentflow into electric energy storage cell stack number 4 may be reduced ascompared to if electric energy storage cell stack number 2 where simplycoupled to electric energy storage cell stack number 4. Consequently, apeak amount of current flow into electric energy storage cell stacknumber four may be further reduced. The voltage level of electric energystorage cell stack number 2 (351) remains unchanged because thecontactor of electric energy storage cell stack number 2 remains open.

The fifth plot 308 from the top of FIG. 3 is a plot of electric energystorage cell stack voltage versus electric energy storage cell stacknumber when the contactors of electric energy storage cell stack numbers4, 1, 3, and 2 are closed. The contactor of electric energy storage cellstack number 2 is closed after the contactor of electric energy storagecell stack number 3 is closed because electric energy storage cell stacknumber 2 has the highest voltage of the four electric energy storagecell stacks at the time when all contactors of all electric energystorage cell stacks were open. Closing the contactor of electric energystorage cell stack number 2 causes the voltage of electric energystorage cell numbers 4 (353), 1 (350), and 3 (352) to increase and thevoltage of electric energy storage cell number 2 (352) to decrease. Thevoltage of electric energy storage cell numbers 4 (353), 1 (350), and 3(352) increases and the voltage of electric energy storage cell number 2(351) decreases because electrical charge is transferred from electricenergy storage cell stack number 2 to electric energy storage cell stacknumbers 1, 4, and 3. The combined voltage of electric energy storagecell stack number 2 and electric energy storage cell stack numbers 4, 1,and 3 raises the voltage of electric energy storage cell stack numbers 1(350), 4 (353), and 3 (352) in a way that reduces current flow intoelectric energy storage cell stack numbers 4 and 1 as compared to if allcontactors of electric energy storage cell stacks 1-4 were closedcontemporaneously. Consequently, the possibility of contactordegradation may be reduced.

In this way, contactors of electric energy storage cells may be closedsequentially in a one after the other fashion so that potential orvoltages of electric energy storage cell stacks with lower voltagelevels may be increased gradually. This may provide a reduction in peakcurrent flow into electric energy storage cell stacks that were at lowervoltages when the process started as compared to conditions where allcontactors of all electric energy cell stacks are contemporaneouslyclosed. Thus, contactors of the individual electric energy storage cellstacks are controlled independently and according to voltages of theelectric energy storage cell stacks to reduce current flow into electricenergy storage cell stacks that exhibit lower open circuit voltages.

Referring now to FIG. 4, a second prophetic example of operating anelectric energy storage system according to the method of FIG. 5 isshown. FIG. 4 shows ten plots of control parameters during an electricenergy storage system operating sequence. The plots occur at the sametime and the plots are time aligned. Vertical lines t0-t6 representtimes of interest in the sequence.

The first plot from the top of FIG. 4 is a plot of the electric energystorage system operating state versus time. The vertical axis representselectric energy storage system operating state and the electric energystorage system is activated when trace 402 is at a higher level near thevertical axis arrow. The electric energy storage system is deactivatedwhen trace 402 is at a lower level near the horizontal axis. Thehorizontal axis represents time and time increase from the left side ofthe plot to the right side of the plot. Trace 402 represents theoperating state of the electric energy storage system.

The second plot from the top of FIG. 4 is a plot of voltage of electricenergy storage cell stack number one (e.g., as determined at an electricenergy storage cell with a highest potential voltage in electric energystorage cell stack number one) versus time. The vertical axis representsvoltage of electric energy storage cell stack number one and voltage ofelectric energy storage cell stack number one increases in the directionof the vertical axis arrow. The horizontal axis represents time and timeincrease from the left side of the plot to the right side of the plot.Trace 404 represents the voltage of electric energy storage cell stacknumber one.

The third plot from the top of FIG. 4 is a plot of the operating stateof the contactor of or associated with (e.g., a contactor thatselectively couples the electric energy storage cell stack to theelectric power conductor or bus 260) electric energy storage cell stacknumber one versus time. The vertical axis represents the operating stateof the contactor of electric energy storage cell stack number one andthe contactor of electric energy storage cell stack number one is closedwhen trace 406 is at a higher level near the vertical axis arrow. Thecontactor of electric energy storage cell stack number one is open whentrace 406 is at a lower level near the horizontal axis. The horizontalaxis represents time and time increase from the left side of the plot tothe right side of the plot. Trace 406 represents the operating state ofthe contactor of electric energy storage cell stack number one.

The fourth plot from the top of FIG. 4 is a plot of voltage of electricenergy storage cell stack number two (e.g., as determined at an electricenergy storage cell with a highest potential voltage in electric energystorage cell stack number two) versus time. The vertical axis representsvoltage of electric energy storage cell stack number two and voltage ofelectric energy storage cell stack number two increases in the directionof the vertical axis arrow. The horizontal axis represents time and timeincrease from the left side of the plot to the right side of the plot.Trace 408 represents the voltage of electric energy storage cell stacknumber two.

The fifth plot from the top of FIG. 4 is a plot of the operating stateof the contactor of or associated with (e.g., selectively couples theelectric energy storage cell stack to the electric power conductor orbus 260) electric energy storage cell stack number two versus time. Thevertical axis represents the operating state of the contactor ofelectric energy storage cell stack number two and the contactor ofelectric energy storage cell stack number two is closed when trace 410is at a higher level near the vertical axis arrow. The contactor ofelectric energy storage cell stack number two is open when trace 410 isat a lower level near the horizontal axis. The horizontal axisrepresents time and time increase from the left side of the plot to theright side of the plot. Trace 410 represents the operating state of thecontactor of electric energy storage cell stack number two.

The sixth plot from the top of FIG. 4 is a plot of voltage of electricenergy storage cell stack number three (e.g., as determined at anelectric energy storage cell with a highest potential voltage inelectric energy storage cell stack number three) versus time. Thevertical axis represents voltage of electric energy storage cell stacknumber three and voltage of electric energy storage cell stack numberthree increases in the direction of the vertical axis arrow. Thehorizontal axis represents time and time increase from the left side ofthe plot to the right side of the plot. Trace 412 represents the voltageof electric energy storage cell stack number three.

The seventh plot from the top of FIG. 4 is a plot of the operating stateof the contactor of or associated with (e.g., selectively couples theelectric energy storage cell stack to the electric power conductor orbus 260) electric energy storage cell stack number three versus time.The vertical axis represents the operating state of the contactor ofelectric energy storage cell stack number three and the contactor ofelectric energy storage cell stack number three is closed when trace 414is at a higher level near the vertical axis arrow. The contactor ofelectric energy storage cell stack number three is open when trace 414is at a lower level near the horizontal axis. The horizontal axisrepresents time and time increase from the left side of the plot to theright side of the plot. Trace 414 represents the operating state of thecontactor of electric energy storage cell stack number three.

The eighth plot from the top of FIG. 4 is a plot of voltage of electricenergy storage cell stack number four (e.g., as determined at anelectric energy storage cell with a highest potential voltage inelectric energy storage cell stack number four) versus time. Thevertical axis represents voltage of electric energy storage cell stacknumber four and voltage of electric energy storage cell stack numberfour increases in the direction of the vertical axis arrow. Thehorizontal axis represents time and time increase from the left side ofthe plot to the right side of the plot. Trace 416 represents the voltageof electric energy storage cell stack number four.

The ninth plot from the top of FIG. 4 is a plot of the operating stateof the contactor of or associated with (e.g., a contactor thatselectively couples the electric energy storage cell stack to theelectric power conductor or bus 260) electric energy storage cell stacknumber four versus time. The vertical axis represents the operatingstate of the contactor of electric energy storage cell stack number fourand the contactor of electric energy storage cell stack number four isclosed when trace 418 is at a higher level near the vertical axis arrow.The contactor of electric energy storage cell stack number four is openwhen trace 418 is at a lower level near the horizontal axis. Thehorizontal axis represents time and time increase from the left side ofthe plot to the right side of the plot. Trace 418 represents theoperating state of the contactor of electric energy storage cell stacknumber four.

The tenth plot from the top of FIG. 4 is a plot of the operating stateof the main contactor of the electric energy storage system versus time.The vertical axis represents the operating state of the main contactorof the electric energy storage system and the main contactor of theelectric energy storage system is closed when trace 420 is at a higherlevel near the vertical axis arrow. The main contactor of the electricenergy storage system is open when trace 420 is at a lower level nearthe horizontal axis. The horizontal axis represents time and timeincrease from the left side of the plot to the right side of the plot.Trace 420 represents the operating state of the main contactor of theelectric energy storage system.

At time t0, the electric energy storage system is off and the contactorsof all electric energy storage cell stacks are open, therebyelectrically isolating the electric energy storage cell stacks fromelectric power conductor or bus 260. The voltage of electric energystorage cell stack number three (412) is the lowest voltage of theelectric energy storage cell stacks. The voltage of electric energystorage cell stack number four (416) is the highest voltage of theelectric energy storage cell stacks. The main output contactor is opento electrically isolate the electric energy storage system from externalelectrical sources and consumers.

At time t1, the electric energy storage system is activated and thecontactors of all electric energy storage cell stacks are still open,but shortly thereafter, the contactor of electric energy storage cellstack number three is closed in response to a request to via theelectric energy storage system to close contactors of the electricenergy storage cell stacks because the voltage of electric energystorage cell stack number three (412) is the lowest voltage of theelectric energy storage cell stacks. Closing the contactor of electricenergy storage cell stack number three electrically couples electricenergy storage cell stack number three to the electric power conductoror bus 260. The contactors of energy storage cell stack numbers one,two, and four remain open. The voltages of the energy storage cellstacks remain unchanged and the main contactor remains in an open state.In this example, electric energy storage cell stack number three is notcharged after its contactor is closed because its voltage is within athreshold voltage of the electric energy storage cell stack with alowest voltage and open contactor (e.g., electric energy storage cellstack number one).

At time t2, the electric energy storage system remains activated and thecontactor of electric energy storage cell stack number one is closed inresponse to a request of the electric energy storage system to closecontactors of the electric energy storage cell stacks and a voltage ofelectric energy storage cell stack number one (404) being within athreshold voltage of the voltage of electric energy storage cell stacknumber three (412). Shortly after time t2, the voltage of electricenergy storage cell stack number one (404) is reduced by a small amountand voltage of electric energy storage cell stack number three (412) isincreased by a small amount as change is transferred from electricenergy storage cell stack number one to electric energy storage cellstack number three. Number three and number one electric energy storagecell stacks are now electrically coupled to the electric power conductoror bus 260. The contactors of energy storage cell stack numbers two andfour remain open. The voltages of the energy storage cell stack numberstwo and four remain unchanged and the main contactor remains in an openstate.

At time t3, electric energy storage cell stack numbers three and onebegin to be charged (e.g., receive charge) from a charger that iselectrically coupled to the electric power conductor or bus 260.Electric energy storage cell stack numbers three and one are chargedbecause the voltages of electric energy storage cell stack number twoand the voltage of electric energy storage cell stack number four arenot within a threshold voltage of the voltage of bus 260 (e.g., thevoltage of electric energy storage cell stack numbers one and three).The voltages of the energy storage cell stack numbers two and fourremain unchanged and the main contactor remains in an open state.

At time t4, the electric energy storage system remains activated and thecontactor of electric energy storage cell stack number two is closed inresponse to a request of the electric energy storage system to closecontactors of the electric energy storage cell stacks and a voltage ofelectric energy storage cell stack number two being within a thresholdvoltage of the voltage of electric energy storage cell stack numbers oneand three. Shortly after time t4, the voltage of electric energy storagecell stack number two (408) is reduced by a small amount and voltage ofelectric energy storage cell stack numbers one (404) and three (412) areincreased by a small amount as change is transferred from electricenergy storage cell stack number two to electric energy storage cellstack numbers one and three. Electric energy storage cell stack numberstwo, three and one are now electrically coupled to the electric powerconductor or bus 260. The contactor of energy storage cell stack numberfour remains open. The voltages of the energy storage cell stack numbersfour remains unchanged and the main contactor remains in an open state.

Between time t4 and time t5, electric energy storage cell stack numbersone, three, and two begin to be charged from a charger that iselectrically coupled to the electric power conductor or bus 260. Numberone, two, and three electric energy storage cell stacks are chargedbecause the voltage of electric energy storage cell stack number four isnot within a threshold voltage of the voltage of conductor or bus 260(e.g., the voltage of electric energy storage cell stack numbers one,two, and three). The voltage of electric energy storage cell stacknumber four remains unchanged and the main contactor remains in an openstate.

At time t5, the electric energy storage system remains activated and thecontactor of electric energy storage cell stack number four is closed inresponse to a request of the electric energy storage system to closecontactors of the electric energy storage cell stacks and a voltage ofelectric energy storage cell stack numbers four being within a thresholdvoltage of the voltage of electric energy storage cell stack numbersone, two, and three. Shortly after time t5, the voltage of electricenergy storage cell stack number four (416) is reduced by a small amountand voltage of electric energy storage cell stack numbers one, three,and two are increased by a small amount as change is transferred fromelectric energy storage cell stack number four to electric energystorage cell stack numbers one (404), three (414), and two (408). Allelectric energy storage cell stacks are now electrically coupled to theelectric power conductor or bus 260. The main contactor remains in anopen state and all the electric energy storage cell stacks are chargedvia the charger.

At time t6, the main contractor is closed and the charger is deactivatedwhen charge of all electric energy storage cell stacks are within athreshold voltage of a desired voltage. All of the contactors of all ofthe electric energy storage cell stacks remain in closed states so thatexternal sources may supply electrical energy from electric energystorage system 200 or so that external electric power consumers may drawelectrical energy from electric energy storage system 200.

In this way, contactors of electric energy storage cell stacks may beclosed and charge may be applied to electric energy storage cell stacksso that when one or more contactors of other electric energy storagecell stacks close, current flow throughout the electric energy storagesystem may be below desired levels. Further, when there is littledifference between levels of charge of electric energy storage cellstacks, contactors of electric energy storage cell stacks may be closedwithout providing charge to electric energy storage cell stacks via thecharger.

Referring now to FIG. 5, a method for operating the electric energysystem as shown in FIGS. 1 and 2 is shown. The method of FIG. 5 may beincluded as executable instructions stored in non-transitory memory ofthe system of FIGS. 1-2. In addition, the methods of FIG. 5 may work incooperation with the system of FIGS. 1 and 2 to receive data and adjustactuators to control the system of FIGS. 1 and 2.

At 502, method 500 judges if operation of the electric energy storagesystem is requested. Operation of the electric energy storage system maybe requested via input to a human/machine interface, a controller, orother source. If method 500 judges that operation of the electric energystorage system is requested, the answer is yes and method 500 proceedsto 504. Otherwise, the answer is no and method 500 proceeds to 550.

At 550, method 500 deactivates electrolyte pumps to conserve energy. Theelectrolyte pumps may be deactivated via ceasing to supply energy to theelectrolyte pumps. Method 500 proceeds to 552.

At 552, method 500 opens one or more main or output contactors toelectrically decouple the electric energy storage system from externalpower sources or consumers. In addition, method 500 opens all the of thecontactors of the electric energy storage cell stacks so that theelectric energy storage cell stacks are electrically decoupled from eachother. Method 500 proceeds to exit.

At 504, method 500 activates electrolyte pumps to operate the electricenergy storage cells. The electrolyte pumps may be activated viasupplying energy to the electrolyte pumps. Method 500 proceeds to 506.

At 506, method 500 judges if the electric energy storage system hasrequested closing all contactors of or associated with (e.g., acontactor that selectively couples the electric energy storage cellstack to the electric power conductor or bus 260) electric energystorage cell stacks. The electric energy storage system may request thatall contactors of or associated with electric energy storage cell stacksbe closed after the electric energy storage system is activated. Ifmethod 500 judges that the electric energy storage system has requestedclosing all contactors of or associated with electric energy storagecell stacks, then the answer is yes and method 500 proceeds to 508.Otherwise, the answer is no and method 500 returns to 502. At this time,the contactors of each electric energy storage cell stack in a pluralityof electric energy storage cell stacks within the electric energystorage system are open.

At 508, method 500 determines voltages of all electric energy cellstacks. In one example, controller 150 shown in FIGS. 1 and 2 samplesthe highest potential (e.g., voltage) electric energy storage cells ofeach electric energy storage cells stack. If all the contactors of eachelectric energy storage cell stack in a plurality of electric energystorage cell stacks within the electric energy storage system are open,then the electric energy storage cell stacks are electrically de-coupledfrom each other and the controller receives open circuit voltages ofeach electric energy storage cell stack. Method 500 stores each voltageof each electric energy storage cell stack in controller memory.Further, method 500 determines which, if any, contactors of orassociated with electric energy storage cell stacks is open. Method 500may determine which contactors of or associated with electric energystorage cell stacks are open via values of variables stored incontroller memory that indicate whether or not the controller hascommanded the contactors open or closed.

Method 500 may arrange a list of electric energy storage cell stacks incontroller memory by number or location of the electric energy storagecell stack in the electric energy storage system and by the voltages ofthe electric energy storage cell stacks. The list may order the electricenergy storage cell stacks beginning with the electric energy storagecell stack with the lowest voltage to the electric energy storage cellstack with the highest voltage. For example, if the electric energystorage system includes four electric energy storage cell stacks withthe following voltages: electric energy storage cell stack number 1 (4volts); electric energy storage cell stack number 2 (5 volts); electricenergy storage cell stack number 3 (1.5 volts); and electric energystorage cell stack number 4 (3 volts), then method 500 may arrange alist of the cell stacks as follows: electric energy storage cell stacknumber 3; electric energy storage cell stack number 4; electric energystorage cell stack number 1; and electric energy storage cell stacknumber 2. This arrangement of cell stacks may be the basis foractivating contactors of or associated with the electric energy storagecell stacks. In particular, the contactors may be closed according tothe order of electric energy storage cell stack numbers in the list. Thelist may be stored in an area of controller memory.

In another example, Method 500 may arrange a list of groups of electricenergy storage cell stacks in controller memory by the voltage ranges ofthe electric energy storage cell stacks. The list may order the groupsof electric energy storage cell stacks in an order from the electricenergy storage cell stack group with the lowest voltage range to theelectric energy storage cell stack group with the highest voltage. Forexample, if the electric energy storage system includes six electricenergy storage cell stacks with the following voltages: electric energystorage cell stack number 1 (4 volts); electric energy storage cellstack number 2 (5 volts); electric energy storage cell stack number 3(1.5 volts); electric energy storage cell stack number 4 (3 volts),electric energy storage cell stack number 5 (6 volts), and electricenergy storage cell stack number 6 (1 volt), and where the voltagesincluded in the groups are 0-2 volts (first range); 2-4.5 volts (secondrange); and 4.5-7 volts (third range), then method 500 may arrange alist or order of the cell stacks as follows: group 1 (cell stack numbers3 and 6); group 2 (cell stack numbers 1 and 4); group 3 (cell stacknumbers 2 and 5). The contactors of the electric energy storage cellstacks may then be activated according to the order of groups beginningfrom group 1 and ending with group 3. The groups may be stored in a listin an area of controller memory, and membership in a group is based onelectric energy storage cell stack voltage. Method 500 proceeds to 510.

At 510, method 500 judges if any contactors of or associated withelectric energy storage cell stacks are closed in the electric energystorage system. If so, the answer is yes and method 500 proceeds to 511.Otherwise, the answer is no and method 500 proceeds to 512. If theanswer is yes, electric energy storage cell stacks that are electricallycoupled to the electric power conductor or bus 260 may be charged towithin a threshold voltage of a electric energy storage cell stack thatis not electrically coupled to the electric power conductor or bus 260.Alternatively, electric energy storage cell stacks that are electricallycoupled to the electric power conductor or bus 260 may be charged topredetermined levels. The predetermined levels may be based on expectedcurrent flow through the contactors.

At 512, method 500 closes an open contactor of or associated with theelectric energy storage cell stack that exhibits the lowest voltage ofelectric energy storage cell stacks that have open contactors. Forexample, if the electric energy storage system includes four electricenergy storage cell stacks arranged in a list with an order of electricenergy storage cell stack number 3; electric energy storage cell stacknumber 4; electric energy storage cell stack number 1; and electricenergy storage cell stack number 2 as described at 508, then thecontactor of electric energy storage cell stack number 3 may be closedif no other contactors of electric energy storage cell stacks areclosed. If the contactors for electric energy storage cell stack numbers3 and 4 are closed, then the contactor of electric energy storage cellstack number 1 may be closed.

Alternatively, if the electric energy storage cell stacks are arrangedin groups, then method 500 closes open contactors of or associated withthe group of electric energy storage cell stack that includes the lowestvoltages of electric energy storage cell stacks that have opencontactors. For example, if the electric energy storage system includesthree groups of electric energy storage cell stacks arranged in a listwith an order of group 1 (cell stack numbers 3 and 6); group 2 (cellstack numbers 1 and 4); group 3 (cell stack numbers 2 and 5), then thecontactor of electric energy storage cell stacks in group 1 may beclosed if no other contactors of electric energy storage cell stacks areclosed. If the contactors for group 1 of the electric energy storagecell stack groups are closed, then the contactor of group 2 of theelectric energy storage cell stack groups may be closed since group 2includes electric energy storage cell stacks having the lowest voltagesof electric energy storage cell stacks with open contactor. Method 500proceeds to 514.

At 514, method 500 activates a charger and charges electric energystorage cell stacks. In particular, method 500 charges electric energycell stacks that are electrically coupled to the electric powerconductor or bus 260. The charger may charge the electric energy storagecell stacks that are coupled to the electric power conductor or bus 260to a voltage that is within a threshold voltage of an electric energystorage cell stack that has an open contactor and lowest voltage amongelectric energy storage cell stacks that have open contactors.Alternatively, charger may charge the electric energy storage cellstacks that are coupled to the electric power conductor or bus 260 to apredetermined voltage. Further, if the electric energy storage cellstacks that are coupled to the electric power conductor or bus 260 arewithin a threshold voltage of an electric energy storage cell stack thathas an open contactor and lowest voltage among electric energy storagecell stacks that have open contactors, then the charger may not beactivated and may not supply charge to the electric energy storage cellstacks that are coupled to the electric power conductor or bus 260.Method 500 proceeds to 516.

At 516, method judges if all contactors of or associated with electricenergy storage cell stacks are closed in the electric energy storagesystem. In one example, method 500 may judge that all contactors of orassociated with electric energy storage cell stacks are closed in theelectric energy storage system based on values of variables stored incontroller memory. Alternatively, the contactors may directly indicatetheir operating state to the controller so that the controller may makethe determination. If method 500 judges that all contactors of orassociated with electric energy storage cell stacks in the electricenergy storage system are closed, then the answer is yes and method 500proceeds to 518. Otherwise, the answer is no and method 500 returns to508.

At 518, method 500 closes the main contactor of the electric energystorage system so that the electric energy storage cell stacks arecoupled to external electrical power consumers and sources. Further,method 500 may deactivate the charger at 518. Method 500 proceeds toexit.

At 511, method 500 judges if a voltage of an electric energy storagecell stack with a lowest voltage among electric energy storage cellstacks with open contactors is within a desired range of a thresholdvoltage. If so, the answer is yes and method 500 proceeds to 512.Otherwise, the answer is no and method 500 proceeds to 514.

In one example, method 500 may charge electric energy storage cellsstacks that are coupled to the electric power conductor 260 when avoltage of an electric energy storage cell stack that is notelectrically coupled to the electric power conductor 260 is not within adesired range. The desired range may be a voltage range that is within athreshold voltage of electric energy storage cell stacks that areelectrically coupled to the electric power conductor 260. For example,if the desired range is 5 volts and electric energy storage cells stacksthat are electrically coupled to the electric power conductor 260 are at50 volts while an electric energy storage cells stack with a lowestvoltage that is not electrically coupled to the electric power conductor260 output voltage is 65 volts, then the answer is no and method 500proceeds to 514. However, if the output voltage of the electric energystorage cells stack with the lowest voltage that is not electricallycoupled to the electric power conductor 260 is 54 volts and within 5volts of the 50 volts of electric energy storage cells stacks that areelectrically coupled to the electric power conductor 260, then thecharger may not charge electric energy storage cells stacks that areelectrically coupled to the electric power conductor 260.

Alternatively, method 500 may charge electric energy storage cellsstacks that are coupled to the electric power conductor 260 when avoltage of a group of electric energy storage cell stacks that are notelectrically coupled to the electric power conductor 260 and that have alowest voltage level of groups that are not electrically coupled to theelectric power conductor 260 are not within a desired range of thevoltage of electric energy storage cells stacks that are coupled to theelectric power conductor 260. The desired range may be a voltage rangethat is within a threshold voltage of electric energy storage cellstacks that are electrically coupled to the electric power conductor260. For example, if the desired range is 5 volts and electric energystorage cells stacks that are electrically coupled to the electric powerconductor 260 are at 50 volts while a group of electric energy storagecells stack with a lowest voltage of groups that are not electricallycoupled to the electric power conductor 260 output voltage is 65 volts,then the answer is no and method 500 proceeds to 514. However, if theoutput voltage of the group of electric energy storage cells stack withthe lowest voltage that is not electrically coupled to the electricpower conductor 260 is 52 volts and within 5 volts of the 50 volts ofelectric energy storage cells stacks that are electrically coupled tothe electric power conductor 260, then the charger may not chargeelectric energy storage cells stacks that are electrically coupled tothe electric power conductor 260.

In this way, method 500 may raise voltages or charge of electric energystorage cell stacks that coupled to electric power conductor 260 so thatwhen contactors or groups of contactors that are open are subsequentlyclosed, current flow into electric energy storage cell stacks that arecoupled to electric power conductor 260 is not greater than is desired.

Thus, the method of FIG. 5 provides for a method for operating anelectrical energy storage system, comprising: sequentially closing aplurality of contactors that selectively electrically couple a pluralityof electrical energy storage cell stacks to an electric power conductorwithin the electrical energy storage system according to individualvoltages of each of the plurality of electric energy storage cell stacksvia a controller. The method includes where sequentially closing aplurality of contactors that selectively couple a plurality of electricenergy storage cell stacks to an electric power conductor within theelectrical energy storage system according to individual voltages ofeach of the plurality of electric energy storage cell stacks includesclosing the plurality of contactors in an order beginning with an energycell stack of the plurality of cell stacks having a voltage less than orequal to voltages of all other energy cell stacks included in theplurality of electric energy storage cell stacks, the order ending withan energy cell stack of the plurality of cell stacks having a voltagegreater than or equal to voltages of all other energy cell stacksincluded in the plurality of electric energy storage cell stacks.

The method of FIG. 5 further comprises determining voltages of theplurality of electric energy storage cell stacks via the controller. Theelectric power system further comprises charging one or more of theplurality of electric energy storage cell stacks via a charger during atime between closing a first of the plurality of contactors and a secondof the contactors. The electric power system includes where the chargeris coupled to the electric power conductor. The electric power systemfurther comprises charging the one or more of the plurality of electricenergy storage cell stacks to a threshold voltage before closingadditional contactors included in the plurality of contactors. Themethod includes where the threshold voltage is based on a voltage of oneor more of the plurality of electric energy storage cell stacks thathave not been coupled to the electric power conductor. The method alsoincludes where the threshold voltage is a predetermined voltage.

The method of FIG. 5 also provides for a method for operating anelectrical energy storage system, comprising: sensing voltages of eachof a plurality of electric energy storage cell stacks via a controller;grouping each of the plurality of electric energy storage cell stacksinto more than one groups of electric energy storage cell stacksaccording to the voltages of each of the plurality of electric energystorage cell stacks; and via the controller, sequentially coupling eachof the more than one groups of electric energy storage cell stacks to anelectric power conductor within the electrical energy storage systemaccording to voltages of electric energy storage cell stacks included inthe more than one groups of electric energy storage cell stacks. Themethod includes where the more than one group of electric energy storagecell stacks are based on voltage ranges.

In some examples, the method includes where membership of an electricenergy storage cell stack included in the plurality of electric energystorage cell stacks into one of the more than one groups of electricenergy storage cell stacks is based on a voltage of the electric energystorage cell stack. The method further comprises charging one or more ofthe plurality of electric energy storage cell stacks via a chargerduring a time between coupling a first group of the groups of electricenergy storage cell stacks to the electric power conductor and couplinga second group of the groups of electric energy storage cell stacks tothe electric power conductor. The method further comprises charging theone or more of the plurality of electric energy storage cell stacks to athreshold voltage before coupling the second group of electric energystorage cell stacks to the electric power conductor. The method includeswhere the threshold voltage is based on a voltage of one or more of theplurality of electric energy storage cell stacks that have not beencoupled to the electric power conductor.

Note that the example control and estimation routines included hereincan be used with various electric energy storage system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other system hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, atleast a portion of the described actions, operations and/or functionsmay graphically represent code to be programmed into non-transitorymemory of the computer readable storage medium in the control system.The control actions may also transform the operating state of one ormore sensors or actuators in the physical world when the describedactions are carried out by executing the instructions in a systemincluding the various described hardware components in combination withone or more controllers.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description.

1. A method for operating an electrical energy storage system,comprising: sequentially closing a plurality of contactors thatselectively couple a plurality of electric energy storage cell stacks toan electric power conductor within the electrical energy storage systemaccording to individual voltages of each of the plurality of electricenergy storage cell stacks via a controller.
 2. The method of claim 1,where sequentially closing a plurality of contactors that selectivelycouple a plurality of electric energy storage cell stacks to an electricpower conductor within the electrical energy storage system according toindividual voltages of each of the plurality of electric energy storagecell stacks includes closing the plurality of contactors in an orderbeginning with an energy cell stack of the plurality of cell stackshaving a voltage less than or equal to voltages of all other energy cellstacks included in the plurality of electric energy storage cell stacks,the order ending with an energy cell stack of the plurality of cellstacks having a voltage greater than or equal to voltages of all otherenergy cell stacks included in the plurality of electric energy storagecell stacks.
 3. The method of claim 1, further comprising determiningvoltages of the plurality of electric energy storage cell stacks via thecontroller.
 4. The method of claim 1, further comprising charging one ormore of the plurality of electric energy storage cell stacks via acharger during a time between closing a first of the plurality ofcontactors and a second of the contactors.
 5. The method of claim 4,where the charger is coupled to the electric power conductor.
 6. Themethod of claim 5, further comprising charging the one or more of theplurality of electric energy storage cell stacks to a threshold voltagebefore closing additional contactors included in the plurality ofcontactors.
 7. The method of claim 6, where the threshold voltage isbased on a voltage of one or more of the plurality of electric energystorage cell stacks that have not been coupled to the electric powerconductor.
 8. The method of claim 6, where the threshold voltage is apredetermined voltage.
 9. A method for operating an electrical energystorage system, comprising: sensing voltages of each of a plurality ofelectric energy storage cell stacks via a controller; grouping each ofthe plurality of electric energy storage cell stacks into more than onegroups of electric energy storage cell stacks according to the voltagesof each of the plurality of electric energy storage cell stacks; and viathe controller, sequentially coupling each of the more than one groupsof electric energy storage cell stacks to an electric power conductorwithin the electrical energy storage system according to voltages ofelectric energy storage cell stacks included in the more than one groupsof electric energy storage cell stacks.
 10. The method of claim 9, wherethe more than one groups of electric energy storage cell stacks arebased on voltage ranges.
 11. The method of claim 10, where membership ofan electric energy storage cell stack included in the plurality ofelectric energy storage cell stacks into one of the more than one groupsof electric energy storage cell stacks is based on a voltage of theelectric energy storage cell stack.
 12. The method of claim 9, furthercomprising charging one or more of the plurality of electric energystorage cell stacks via a charger during a time between coupling a firstgroup of the groups of electric energy storage cell stacks to theelectric power conductor and coupling a second group of the groups ofelectric energy storage cell stacks to the electric power conductor. 13.The method of claim 12, further comprising charging the one or more ofthe plurality of electric energy storage cell stacks to a thresholdvoltage before coupling the second group of electric energy storage cellstacks to the electric power conductor.
 14. The method of claim 13,where the threshold voltage is based on a voltage of one or more of theplurality of electric energy storage cell stacks that have not beencoupled to the electric power conductor.
 15. An electric power system,comprising: a plurality of electric energy storage cell stacks, each ofthe plurality of electric energy storage cell stacks including acontactor, the contactor selectively coupling one of the plurality ofelectric energy storage cell stacks to an electric power conductor inthe electric power system; and a controller electrically coupled to thecontactor of each of the plurality of electric energy storage cellstacks.
 16. The electric power system of claim 15, further comprisingexecutable instructions stored in non-transitory memory of thecontroller to sequentially couple the plurality of electric energystorage cell stacks to the electric power conductor within theelectrical energy storage system according to individual voltages ofeach of the plurality of electric energy storage cell stacks.
 17. Theelectric power system of claim 16, further comprising executableinstructions stored in non-transitory memory of the controller to groupeach of the plurality of electric energy storage cell stacks into morethan one group of electric energy storage cell stacks according tovoltages of each of the plurality of electric energy storage cellstacks.
 18. The electric power system of claim 17, further comprisingadditional instructions to sequentially coupling each of the more thanone group of electric energy storage cell stacks to the electric powerconductor within the electrical energy storage system according tovoltages of electric energy storage cell stacks included in the morethan one group of electric energy storage cell stacks.
 19. The electricpower system of claim 15, further comprising a charger coupled to theelectric power conductor.
 20. The electric power system of claim 19,further comprising executable instructions stored in non-transitorymemory of the controller to charge one or more of the plurality ofelectric energy storage cell stacks.